Diamond, which is excellent in hardness and thermal conductivity, exhibits high performance upon application to a cutting tool or a wear resistant tool, and is employed for various purposes. For example, Japanese Patent Publication No. 52-12126 (1977) describes a diamond cutting tool employing a diamond sintered body, which is prepared by sintering fine grains of diamond with a ferrous metal binder in order to suppress a defect caused by cleavage, which is a disadvantage of monocrystalline diamond, in particular.
However, such a tool employing a diamond sintered body has a low heat resistance which is a problem. In more concrete terms, the tool may deteriorate in wear resistance and strength when the temperature of its cutting edge is increased and lose practicability. For example, the diamond sintered body is reduced in wear resistance and strength when the same is heated to a temperature exceeding 750.degree. C., and broken when the same is further heated to a temperature exceeding 900.degree. C. This is conceivably due to graphitization of the diamond caused at interfaces between the diamond grains and the ferrous metal binder, and a high thermal stress is generated due to the difference in thermal expansion coefficients between the diamond grains and the metal binder under a high temperature.
In order to improve the heat resistance, there has been developed another tool employing a diamond sintered body, as described in Japanese Patent Laying-Open No. 53-114589 (1978), for example. According to this prior art, an acid treatment is performed on the sintered body to remove most portions of a binder metal layer. In this technique, however, the tool is remarkably reduced in strength due to holes caused by the removed portions of the binder metal layer, although the heat resistance is improved.
Japanese Patent Laying-Open No. 59-161268 (1984) or 61-33865 (1986), for example, discloses a tool employing a heat-resistant diamond sintered body not containing any holes, in order to solve the aforementioned problem. Such a heat-resistant diamond sintered body contains a binder of Si, SiC or an Ni-Si alloy. However, this sintered body is insufficient in wear resistance due to a small degree of cohesion between the diamond grains and a large content of the binder. Thus, no satisfactory performance has generally been attained in relation to wear resistance and strength of a diamond tool, in spite of the improvement in heat resistance.
Under such circumstances, there has been developed a cutting tool employing polycrystalline diamond which is synthesized by low-pressure vapor deposition which does not contain any binder, as described in Japanese Patent Laying-Open No. 1-212767 (1989), for example, in order to solve problems in a diamond tool employing a diamond sintered body.
In a cutting tool employing a tool material of polycrystalline diamond, a polycrystalline diamond layer is bonded to a tool holder so that a cutting edge is formed on this polycrystalline diamond layer. In connection with such a cutting tool which is prepared by brazing a tool material of polycrystalline diamond with a tool holder, however, the inventors have found that not only the performance of the tool material itself but that of a brazing layer for bonding the tool material to the tool holder are factors controlling the performance of the tool. The inventors have also found that the heat resistance and the thickness of the brazing layer remarkably influence the tool performance in addition to the thickness of the polycrystalline diamond layer, particularly when the polycrystalline diamond layer is not more than 0.5 mm in thickness.
In other words, the strength of the tool is lowered as the thickness of the polycrystalline diamond layer is reduced, although the polycrystalline diamond layer is preferably minimized in thickness in view of the production cost. Even if the thickness of the polycrystalline diamond layer exerts no influence on the tool strength, the brazing layer may be remarkably deformed particularly when the cutting edge is heated to a high temperature if the brazing layer has an inferior heat resistance or an excessive thickness, whereby the strength of the overall tool is reduced.
On the other hand, polycrystalline diamond which is synthesized by low-pressure vapor deposition has an excellent strength, wear resistance and heat resistance since the same is substantially formed only of diamond, and is regarded as an ideal material for a diamond tool.
In a cutting tool made of a polycrystalline diamond material which is synthesized by low-pressure vapor deposition, however, a work piece particles may be deposited on the tool depending on cutting conditions, leading to a problem of an insufficient work surface roughness.